Typical prior art power control circuits employ an integrated circuit, along with several external components, to control the duty cycle of a power switching transistor. The duty cycle is defined as a ratio between on-time or pulse-time and duration of trigger period. The duty cycle of the power switch is used to control the output voltage waveform so that it approximates the shape of the input voltage waveform. The integrated circuits used in prior art power controllers generally include a pulse-width-modulator (PWM) circuit.
In the conventional prior art embodiment, a triangular wave is generated to compare with the input signal and, when amplified, results in a rectangular wave output in which the duty cycle is somewhat proportional to the input signal. There are multitude of drawbacks to this approach. One problem is that the output duty cycle does not accurately represent the input signal due to the fact that the triangular wave utilized for comparison is imperfect. Another problem is that there is no feedback to correct these errors.
Furthermore, a true pulse-width-modulator (PWM) circuit having a constant frequency, demands that the pulses become extremely narrow during very high (&gt;90%) or very low (&lt;10%) output average voltages, resulting in two problems: (1) lowered efficiency and (2) increased EMI (electromagnetic interference).
What is needed is a simple and efficient PWM in order to increase the accuracy and efficiency of the power control system, while reducing the overall cost.